Discrete components having a varistor function can be used as protective components against ESD (electrostatic discharge). One possible use arises e.g. in radio-frequency filters for mobile radio, for example, in the frequency range of 0.5-5 GHz for the protection of these filters or for the protection of electronics connected downstream or upstream such as signal amplifiers, for example.
Alongside the protection function against ESD, these components can at the same time also ensure a protection function against EMI. A so-called EMI-ESD protective component is thus involved.
One of the most important properties of varistors is the so-called reverse current, which also flows below an overvoltage that is to be conducted away between the varistor electrodes. An excessively high reverse current would lead to a limitation of the functionality; particularly in the case of portable devices, such as a mobile phone, for example, it would lead to excessively rapid rechargeable battery discharge. Furthermore, it is necessary to ensure sufficient stability of the electrical properties during the lifetime of the component and in particular of the device into which the component is incorporated.
Particularly in the case of portable devices, a miniaturization of the components used and in particular an integration of different component functions in a common component are striven for. However, it has not been possible heretofore to integrate varistors into monolithic ceramic multilayer components without having to accept excessively high degradation of the electrical properties of the varistor in the process. Varistor ceramics are sensitive to diffusion with respect to the composition and have therefore not been able hitherto to be co-sintered together with other ceramics in sintered fashion to form a monolithic component.
German patent document DE 102 006000935 A1 discloses a method by which ceramic functional layers can be co-sintered together with tensioning layers on the basis of a glass ceramic with little warpage to form a monolithic multilayer component. An impermissible degradation of the varistor ceramic also occurs during sintering.